The present invention relates generally to the art of welding-type power supplies. More specifically, it relates to welding-type power supplies with thermal protection of switches.
There is a wide variety of welding-type power supplies, many use switches to control power. For example, inverter-based power supplies use two or four switches that are pulse width modulated and invert a bus to provide ac regulated power. Many other topologies also use switches.
It is known in the prior art that power switches can overheat. For example, many switches are rated to have a maximum junction temperature of 150xc2x0 C. Additionally, optimum use of the switch may occur when the temperature is below the maximum, for example at a junction temperature of 130xc2x0 C. Thus, some welding-type power supplies have a thermal management scheme to prevent switch overheating.
Some welding-type power supplies with thermal management measure the temperature at the switch casing or on the heat sink, and compare that to a threshold. Because the temperature is at the heat sink or switch casing, the measured temperature is less than the actual junction temperature. Prior art thermal management schemes attempt to account for the difference by determining the maximum difference between actual and measured temperature (which occurs at maximum switch current). The difference between junction and switch casing temperature for the maximum current (or any given current) may be calculated using the thermal impedance (provided by the switch manufacturer) for a given switch power (Vsat at the given current multiplied by the given current). This difference, is then used to set a maximum sensed temperature, above which the machine output is reduced (to a lessor value or to zero), to protect the switches.
For example, if the maximum junction temperature is 130xc2x0 C., and the determined difference at the maximum machine current is 40xc2x0 C., then the threshold is set to 90xc2x0 C. for all currents. While this adequately protects the switches it overestimates the difference between the junction temperature and the sensed temperature at less than the machine maximum current. The output will be reduced or folded back even if the junction temperature is below the optimal temperature.
Accordingly, a welding-type power supply with thermal management that more closely uses the optimal temperature across a wider operating range is desired.
According to a first aspect of the invention a welding-type power supply includes a power circuit, a temperature sense circuit, a switch feedback circuit, and a controller. The power circuit has at least one switch, and the switch has a control input. The temperature sense circuit is disposed to sense a temperature produced by the switch, and has an output indicative of the sensed temperature. The switch feedback circuit senses at least one operating parameter of the switch, and provides an output responsive to the switch parameter. The controller includes a temperature circuit that provides a control output to the switch in response to the switch feedback output and the temperature feedback.
According to a second aspect of the invention a method of providing welding-type power includes controlling a power circuit. The power circuit has one or more switches with control inputs. A temperature produced by the switch or switches is sensed, as is at least one switch operating parameter. The output of the power circuit is reduced if the sensed temperature exceeds a corrected threshold. The corrected threshold is responsive to the operating parameter.
The operating parameter of the switch (or switches) includes current or a function thereof, voltage or a function thereof, or a combination thereof, in various embodiments.
The welding-type power supply includes a look-up table that has the operating parameter as an index, and temperature adjust values and/or corrected threshold values stored as data associated with the index, in one embodiment.
The welding-type power supply includes a calculation circuit that calculates a temperature adjust value from the switch parameter, in another embodiment.
The stored data and/or the calculated values include temperature corrections based on switch current and switch voltage in another embodiment.
The welding-type power supply includes a thermal resistor to provide the temperature feedback in one alternative.
Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description and the appended claims.